1. Field of the Invention
This invention pertains to a process for the inversion polymerization of vinylchloride; and, more specifically, is directed to said polymerization process conducted in a reactor whose internal surfaces are treated with a single coating agent comprising a clay and a water-soluble suspending agent.
2. Description of the Prior Art
As described in, e.g., British Patent No. 748,727, an inversion polymerization comprises two processes: the initial polymerization process for the conversion of the vinylchloride monomer, e.g., to about 20% by mass in the oil phase, the monomer being the continuum; and the second polymerization process for further conversion of the monomer, e.g., up to 95% by mass in the aqueous phase, water being the continuum. The phase inversion occurs as a result of adding a sufficient amount of water to the reaction medium, when the level of polymerization reaches a certain level, e.g., 20%, thereby causing or requiring a dispersion of the unpolymerized monomer particles in the water. Such inversion polymerization has been developed and favored as it is capable of producing polymers with improved uniform characteristics such as porosity, size and structure.
It is, however, known that such an inversion process is often hampered by an excessive polymer buildup on the internal surfaces of the reactor, which often leads to the employment of radical means of removal such as a high pressure injection of water, solvent and various other types of reactor cleaners. Such cleaning process is not only costly due to the labor and the downtime of the reactor; but also presents serious potential health hazards.
Such polymer buildup problem has thus led the industry to search for effective methods of solving the problem. For instance, U.S. Pat. No. 4,117,216 ("'216 process") discloses the idea of coating the inner surface of the reactor with a water-soluble suspending agent. The methods disclosed in U.S. Pat. No. 4,182,808 ("'808 process") and U.S. Pat. No. 4,182,809 ("'809 process") attempt to further improve the '216 process by way of employing a dual coating system: namely, the use of monomer-soluble chemicals as the primary coating and the use of a water-soluble suspending agent as the top coating. Although the coating methods proposed in these patents reduce the level of polymer buildup, none of them is proven to be completely satisfactory. Furthermore, the '808 and '809 processes are rather cumbersome and impractical as they require an extra coating process and the presence of the residues of the oil-soluble coating agent or the primary coating in the final product is less desirable.